Summary

Guar gum and its derivatives have been the most commonly used polymers to increase the viscosity of fracturing fluids. However, the conductivity of many fractures created with guar-based polymers is low because of residual unbroken polymer gel remaining in the fracture. This residue can cause permeability impairment in the proppant pack, resulting in low fracture conductivity and decreased effective fracture length. In this study, we experimentally evaluated two important aspects of the gel damage process--the thickness of the polymer-gel filter cake that is created as fracture-fluid filtrate leaks off into the formation and the yield stress of the concentrated polymer gel that accumulates in the fracture. The thickness of the filter cake created during the leakoff process was measured as a function of the polymer loading and the volume of leakoff. We created the filter cake following the procedure described by Ayoub et al. (2006) and then measured the filter-cake thickness with a precise laser profilometer. We found that the filter-cake thickness varied linearly with leakoff volume, meaning that the gel concentration factor is constant for this guar polymer fluid. The concentrated polymer filter cakes created by leakoff behave rheologically as Herschel-Bulkley fluids having a yield stress. The yield stress of this material is a critical parameter influencing whether the gel can be removed from the fracture. We measured the yield stress of borate-crosslinked guar polymer fracture fluids at concentrations up to 200 lbm/1,000 gals by use of a unique flat-plate device. The yield stresses of the polymer filter cakes were found to depend strongly on the concentration of both polymer and breaker.